1. Field of the Invention This invention relates to oral compositions, more particularly to oral compositions comprising as an abrasive cleaning agent particles of alumina trihydrate (Al.sub.2 O.sub.3 3H.sub.2 O), sometimes referred to as aluminum trihydrate or aluminum hydroxide.
2. Description of the Related Art
Alumina trihydrate is a well-known abrasive agent of oral compositions such as toothpastes. It is made by the Bayer process which produces a coarse grade of material which is then milled to a particle size appropriate for use in an oral composition.
In GB No. 1 277 585 (Unilever) it is disclosed that toothpastes containing milled alumina trihydrate can cause corrosion of aluminum toothpaste tubes and that sodium monofluorophosphate and other sources of monofluorophosphate ions are effective to inhibit this corrosion. Certain phosphate esters are also disclosed as being effective in GB No. 1 475 252 (Colgate-Palmolive). The presence of certain materials during the milling process as disclosed in GB No. 1 537 823 (British Aluminum Co. Ltd) and GB No. 1 544 537 (Colgate-Palmolive) is said to modify the surface of the milled alumina trihydrate particles and make them more suitable for inclusion in toothpastes contained in aluminum tubes.
In GB No. 2 009 596 (Unilever) there is described a toothpaste containing MFP and alumina trihydrate abrasive but not containing zinc ion-producing compounds, wherein the abrasive consists of a mixture of an alumina trihydrate having an average particle size of from 5 to 13 microns and an alumina trihydrate having an average particle size of less than 1 micron, the weight ratio of the two alumina trihydrates being from 30:70 to 70:30. The sub-micron alumina trihydrate described has an average particle size of 0.2 to 0.8 micron, particularly about 0.5 micron. It is produced commercially as a fine precipitate and not by grinding larger particles.
It is nowadays common to include sodium monofluorophosphate in dentifrices as a source of fluoride to protect the teeth against dental caries.
We have determined that there is some interaction between sodium monofluorophosphate and an alumina trihydrate abrasive resulting in a loss of soluble fluoride on storage of a dentifrice, which is more marked at elevated temperatures. Thus, there may be some loss of anti-caries efficacy of such toothpastes.